Manual reset thermostats are known in the art. Generally, the manual reset thermostats employ a bi-metal disc that snaps between opposite bowed positions, i.e., a concave position and a convex position, in response to a change in temperature, for closing and opening an electric circuit. The manual reset thermostats are normally closed and are snapped open when the bimetal snap disc reaches a predetermined temperature and flexes from a concave position to a convex position. The movement of the bi-metal disc pushes a movable contact away from a fixed contact to open the circuit. The bi-metal disc in a manual reset thermostat is usually so produced that the temperature at which it would automatically reset, namely, return to its normal concave position, is outside the range of temperatures expected in the environment in which the thermostat is intended to be used. Therefore, the bi-metal disc will not return to its normal concave position without manually resetting the switch. Generally, resetting a manual reset switch is achieved by depressing a reset element, for example, a pushbutton, to cause the bimetal snap disc to move back to its normal concave position and cause the movable contact to be in contact with the fixed contact, closing the electric circuit.
Manual reset thermostats with a trip-free mechanism do not permit the electric circuit to be closed upon manual reset if the bimetal disc has not reached a temperature below a predetermined temperature. A trip-free mechanism refers to a mechanism that cannot restrict the normal opening of a thermostat when a manual reset element, such as a pushbutton is depressed when the thermostat has not returned to a normal operating temperature. In other words, in a trip-free operation, the contacts must remain open with the manual reset element depressed and with the thermostat heated to at least an open or a limit temperature. Therefore, the trip-free mechanism serves as an important safety measure to prevent overriding of the designed temperature limit of the thermostat.
U.S. Pat. No. 3,675,178 to Place and assigned to Therm-O-Disc, Incorporated, the assignee of the present application, which is incorporated herein by reference in its entirety, discloses a manual reset thermostat with a trip-free mechanism. The manual reset thermostat includes a bi-metal disc, a pushbutton, and an actuating pin connected to the bi-metal disc for pushing the movable contact away from the fixed contacts when the bi-metal disc snaps open. These components must be precisely configured so that when the pushbutton is depressed, the top surface of the actuating pin engages the bottom surface of the pushbutton and stops the pushbutton from moving further to move the bi-metal disc to its reset position. Unless the temperature has dropped to the normal operating temperature, the bi-metal will not return to its reset position when the pushbutton is depressed.
The trip-free manual reset thermostats disclosed in the prior art, however, can be difficult to manufacture. For example, manual reset thermostats in the prior art rely on the dimensional interrelationship between the actuating pin, the pushbutton, and the bimetal disc, as well as the travel limitation on the pushbutton, to achieve the trip-free function. Given the narrow manufacturing and assembly tolerances that are required for an accurate interaction between these components, unnecessary time must be spent on gauging and match-fitting these finished components. Moreover, if the gauged components are unsuitable for the designed trip-free function, these components must be scrapped, thereby increasing the manufacturing costs.